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Why Logs Check by Tom Gorman, PE, University of Idaho |
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| Dr. Tom Gorman, P.E., is an associate professor at the University of Idaho in Moscow. He teaches courses in wood properties and timber design, and teaches “Understanding Wood” seminars at the Willow Creek Wood Productivity Training Center in Willow Creek, Montana. His research has included log home energy performance and strength properties of logs. He is a licensed professional engineer in Idaho and Washington states. His e-mail address is tgorman@uidaho.edu |  |
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We have all seen the checking that occurs in round logs, heavy timbers, utility poles, and round-wood rustic furniture. Checking is a natural occurrence in wood components that contain the pith, or center of a tree. But what is the cause? As was pointed out in the previous article, wood shrinks twice as much in the tangential direction as it does in the radial direction. This can be observed in the amount of shrinkage that occurs in individual pieces of wood cut from trees. But, whenever concentric circles (continuous growth rings) occur in wood, the circumference of those circles (tangential orientation) shrinks twice as much as does the diameter (radial direction). Since the circumference is shrinking at twice the rate as the diameter, something has to give. |
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| One way to visualize how the difference between tangential and radial shrinkage causes checking in logs is to consider the concentric growth rings. The line formed by each growth ring is basically a series of tangent lines, or, one continuous tangent line (in a manner of speaking). As a log dries, the length of each growth ring will shorten by a proportionate amount (tangential shrinkage). But even though the growth rings are shortening, the overall log diameter shrinks at a lesser rate (radial shrinkage). Stresses build up, and checks occur.
How big will the checks become? It depends on the location within the log. The larger diameter outer rings shrink more than the inner rings, since they all lose a percentage of their overall length. Thus, the checks in smaller logs are narrower than the checks in larger diameter logs, just as checks are narrower the closer one gets to the pith. Checking can be avoided entirely by avoiding what is referred to as “heart-centered” wood, which is wood that contains the pith. Whenever continuous growth rings exist, the tangential versus radial shrinkage stresses will cause checking. Can checking be prevented in round wood? Not really. Some log home manufactures saw a kerf along one edge of a green log before they allow it to dry. This acts as a stress release slot, forcing the check to occur along the saw kerf and preventing additional checks from occurring at random on the rest of the log. Since individual boards cut from logs usually do not contain the pith, these boards will not check during drying. Those boards that do contain the pith will typically check only on one side. So, laminated logs avoid the problem of surface checking. Besides the appearance of checks, which is objectionable to some people, checks serve as water- entry locations for wind-driven rain, causing checked logs to become wetter than those without checks. This may lead to decay problems. Also, checked logs have been found to allow air- infiltration to occur at wall corners, when one end of a check is exposed outdoors and the other end is exposed indoors. These checks should be caulked to improve the energy-efficiency of the home.
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